Sun, the solar system's only star

Introduction
Stars are born. They take shape. They go through a turbulent adolescence, and then they live out their lives in a predictable pattern. Some have companions to provide for. Others rapidly decline and die. In some ways, stars are just like people.

Our star, the Sun, is no exception. Once, people regarded it as a different sort of object than the stars. It ruled the day; stars adorned the night. But over the past few centuries astronomers have come to recognise that it is just one middle-aged member of the vast family of stars. From far away, the Sun would look just like any other star - a point of light. Like any other star it is mortal. The realisation that the Sun is a star has done wonders for astronomy. By studying the closest star, scientists have learned about all stars. Conversely, by studying the stars in all their variety, we have learned about the past and future of our Sun.

Staying Alive
The importance of the Sun to the Earth is one of the main reasons scientists want to understand it. In fact, the impetus for solar science early this century came not from astronomers, but from geologists. At the beginning of this century, they believed that the oldest rocks on the Earth are about 4 billion years old and that the Sun was 4.5 - 5 billion years old. The extreme age came as a surprise. They soon realised that known energy sources could only have kept the Sun alive for 20 million years. Other sources of energy - say, a huge fire - would burn out even quicker. The solution to this age discrepancy was the result of several disparate advances in science.

First, astronomers knew that the Sun has to be extremely hot and dense in its centre if it is to support its own weight. Gas at a high temperature exerts a strong pressure, and this holds up the Sun's outer layers. Second, physicists had recently compared the weight of four atoms of hydrogen with that of one helium atom. Both the hydrogen quadruplet and the helium are composed of essentially the same number of subatomic particles. Yet the helium weighs less. Third, Albert Einstein's new theory of relativity showed that matter can be converted into energy (E=mc2).

At first glance, these three ideas might seem totally unrelated. But from them, they deduced that the Sun's energy source was a process then unknown on Earth: the nuclear fusion of hydrogen to helium. Deep in the Sun's hot and dense core hydrogen atoms are squeezed together or fused into helium atoms. A helium atom has less mass than the hydrogen energy from which it was created and this missing mass turns into energy. Few other methods can generate as much energy as nuclear fusion. A small amount of hydrogen can produce an immense amount of energy - which is why nuclear bombs are so destructive, and why the Sun can keep shining for billions of years.

We are family
How did the Sun become hot and dense to begin with? This is the secret of stellar birth. Though we weren't around to witness the birth of our provider, we can read its early life history in the stars. Specifically, we can look out into space and see new stars being born right now.

The closest example is the Great Nebula in Orion, a pattern of bright stars easily visible to the naked eye. This is a stellar nursery - an enormous, lumpy cloud of cold gas and dust, which turns into hundreds of new blue baby stars. The gas is mostly hydrogen and the dust is something like the dust in a desert storm. Within the clouds are hundreds of condensed, cold lumps of gas and dust. A disturbance, such as a blast wave from a nearby stellar explosion, can cause each lump to begin collapsing under its own weight. When the temperature in the core reached several million degrees, the hydrogen atoms started to fuse together, more energy was released, and so on. A chain reaction started that will go on for billions of years. The outward pressure created by this nuclear fusion counterbalanced the inward pressure of gravity, and when the two cancelled each other out, the lump of dust and gas stopped collapsing. The Sun was born. We can see many examples of such star-forming regions. About two thirds of stars are actually born with nearby twins, but the Sun is alone.

Perfect
Depending on the size of the original lump of gas and dust, the process of stellar birth can give rise to different sorts of stars. A small lump never develops high enough pressures and temperatures to start nuclear fusion. It is doomed to remain a dark, dismal stellar failure - a brown dwarf star. A larger lump becomes a large star, so hot and bright that it burns itself out in a few tens of millions of years. A middle-sized lump, not too small and not too large, becomes a middling star such as the Sun. Which is good: if the Sun had been much smaller, Earth would have been a dark, dead world; much larger and the Earth would have been broiled. Lucky for us, it's the perfect size to sustain life on Earth. In its early years, it went through a tempestuous youth, whipping up strong winds that cleared the solar system of whatever gas had not been incorporated into a planet. But then it settled down. From studying rocks, fossils, and Antarctic ice, scientists think the Sun has been brightening over time, but only slightly. They also estimate it has another 5 billion years to go.

What will happen when the Sun has burnt up all the gas? Fortunately, it will still have reserves of hydrogen in the layers that surround the core. The core will heat up this shell of hydrogen. When the shell gets hot enough to fuse hydrogen to helium, the release of energy will carry on there. But this trick has its price. The source of energy will no longer be the dense, massive core, but rather a shell closer to the surface - and that will make a big (so to speak) difference on the structure of the Sun. It will puff up until its radius is 30 times greater becoming a red giant, similar to the star Arcturus, though much smaller than a supergiant such as Betelguese in the constellation Orion. A red giant is red because its exterior cooled from 9,000 to 3,000 degrees Fahrenheit as it expanded; for a star, red means cool. This red giant stage will last for about 2 billion years.

Smaller
New data from the European Space Agency's Hipparcos satellite has led experts to scale back their estimates of the size of red stars. They now think that the Sun will not engulf us when it becomes a red giant, as previously believed. But this will be small comfort. In its retirement from normal core fusion, our previously nurturing star will care little for its planetary children. It will be pumping out a thousand times more energy, making Earth a good approximation to hell. To add insult to injury, the solar wind (a stream of particles which now gives us fun things like the Northern Lights) will become a cyclone that will make radio communication impossible and perhaps evaporate the atmosphere altogether. Looking on the bright side, the red giant Sun may be warm enough to melt the water-rich, but now frozen moons of Jupiter and Saturn. Humanity, if it is still around, might relocate there.

Meanwhile, what happens to all that helium being produced in the shell? It gently rains onto the dead, but still hot, core of the Sun, making it more massive and more compressed. This raises the temperature of the core until suddenly - and I really do mean suddenly, as in seconds - the helium in the core fires up and begins to fuse itself into carbon.

The End is Nigh (only 5 billion years to go)!
The end is drawing near. Now the Sun has to rearrange its internal structure all over again, as its source of energy is once again the central core. It will contract back to a bit larger than its original radius and will give off ten times as what we are used to now. This phase only lasts another 500 million years, as there are a lot fewer helium nuclei (it took four hydrogen nuclei to make one helium nucleus, and three helium nuclei to make up one carbon nucleus) and the energy production is much less efficient.

As it exhausts the helium in the core, the Sun desperately staves off the inevitable by resorting again to those reserves in its outer layers. Again it expands. This time it grows so large that its outer edge is only weakly gravitationally bound to the core, barely holding itself together anymore. After another 100 million years, things will really start falling apart. The Sun's outer layers, freed from the gravitational clutches of the core, will waft away. Over the course of about 10,000 years, these layers will spread out into space as an enormous sphere of gas lit up by the now naked hot core. These layers constitute a 'planetary nebula', so called because in a small telescope the gas cloud looks a bit like the disc of a planet. The hot core is now a 'white dwarf', a stellar cinder. As a white dwarf, the ex-Sun will glow white-hot for near eternity.

Alas, there will be no dramatic explosions to entertain our distant descendants: the Sun, modest in life, is subdued in death. After the planetary nebula fades, there is no nuclear fusion at all, just a lump of hot soot and some happy memories. The Sun will well and truly be dead.

The Next Generation?
The Sphere of gas drifts off and eventually is gathered up in a new cloud and becomes part of the next generation of star fomation. Maybe one day the ashes of the Sun will throw their lot in with another star to be born, live, die, and, perhaps give sustenance to other warm little planets. Or maybe not.